A method for controlling the quantity of fuel incorporating the following method steps is already known, namely: determining the air-mass flow drawn in by suction by the engine; determining a precontrol variable for the quantity of fuel in dependence upon current values of operating variables of the engine; presetting a desired lambda value for the air/fuel mixture to be supplied to the engine; checking the actual lambda value of the exhaust gas of the engine forward of the catalytic converter; and, controlling the quantity of fuel in dependence upon a comparison of desired and actual lambda values.
It is noted that a desired value is actually pregiven only for continuous controllers and two-step controllers having actual value sensors which measure linearly. In contrast, for two-step controllers having very non-linear sensors, a desired value (here a lambda value) is preset by changing, for example, the switching thresholds, the amplitude of P-jumps or integration speeds and integration times.
An improvement of the above-mentioned method is disclosed in U.S. Pat. No. 5,207,056. The improvement of the known method is provided in this patent in that after a transient disturbance, a control to the desired lambda value is not immediately made; instead, a lambda value disturbance is arbitrarily called up which has an effect which precisely compensates for the effect of the disturbance which was caused by the transient operation. If, for example, the mixture was made too lean because of the transient operation, then an especially large amount of oxygen reaches the catalytic converter and the mixture is so controlled after the decay of the transient operation that the mixture becomes enriched temporarily with respect to the actually wanted desired value one. This enrichment takes place for so long and to such an extent that, because of the fuel supplied in excess, just that oxygen stored in the catalytic converter is combusted which had collected there in the previously-mentioned lean phase.
Furthermore, it is known that the general method mentioned initially can be modified by a second lambda probe mounted rearward of the catalytic converter. A method is described in U.S. Pat. No. 5,018,348 according to which the signal of the rearward lambda probe is utilized to so compensate the signal of the forward lambda probe that the mean value of the signal from the forward lambda probe corresponds to the mean value of the signal from the rearward lambda probe. This measure is based on the realization that a lambda probe mounted rearward of the catalytic converter deteriorates considerably less than a probe mounted forward of the catalytic converter so that the signal from the rearward probe is more reliable over a longer period of time than the signal from the forward lambda probe.
With a probe mounted rearward of the catalytic converter, it is also possible to monitor the oxygen storage capacity of a catalytic converter. Many methods are known for performing this task. All of the methods are based on the consideration that, starting with a catalytic converter which is empty as to oxygen, some time is required after setting a lean mixture until the catalytic converter is filled with oxygen and a lean mixture is measured not only at the input but also at the output of the catalytic converter. And, proceeding from a catalytic converter filled with oxygen, after adjusting a rich mixture, a corresponding time delay occurs between the time points of the announcement of the rich mixture from the forward probe and then from the rearward probe. The time delay or the mean signal amplitudes associated therewith are a measure for the storage capacity of the catalytic converter. Depending upon this storage capacity, it is advantageous to change the operating parameters of the lambda control.
Notwithstanding this change and further changes of the general method described above, it remains impossible even for a new catalytic converter to optimally hold down the exhaust gas emission from the engine having a catalytic converter in all operating conditions, especially for transient operations. Accordingly, the problem existed to provide a method and an arrangement for controlling the quantity of fuel for an internal combustion engine having a catalytic converter which provide still lower emissions of toxic exhaust gas than has previously been possible.